Elimination of spurious response in an electromechanical filter



n ted States ata Q" ELIMINATION-OF SPURIOUS RESPONSE IN AN ELECTROMECHANICAL FILTER Melvin L. Doelz, Glendale, Califl, assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application May 7, 1953, Serial No. 353,643

3 Claims. (Cl. 333-71) This invention relates in. general to electromechanical resonators, and in particular to'means for eliminating spurious responses in such resonators.

Applicant has filed a number of patent applications on electromechanical filters, as for example, see patent applications Serial Number 248,011, filed September 24, 1951, now Patent No. 2,717,361, issued September 6, 1955, and Serial Number 283,340, filed April 21, 1952, now Patent No. 2,693,579, issued November 2, 1954.

These applications disclose electromechanical filters wherein discs are coupled together and energized so that they resonate at a particular frequency to pass energy at that frequency. It has been learned that mechanical resonating systems have a number of modes in which they may vibrate and this gives rise to vibrations at undesired frequencies at times.

It is an object of this invention, therefore, to eliminate spurious frequency responses.

Yet another object of this invention is to provide an electromechanical filter which has spurious frequency response substantially eliminated.

Further objects, features and advantages of the invention will become apparent from the following description and claims when read in view of the drawings, in which:

Figure 1 is a top view of an electromechanical filter;

Figure 2 is a sectional side view of an electromechanical filter;

Figure 3 is a response curve of a normal filter;

2,802,992 Patented Aug. 13, 1957 ported by the stand-cits 11 and 12 immediately above the cylinders 21 and 22. The cylinders 21 and 22 may be made of a non-magnetic material, as for example, brass.

Mounted between the cylinders 21 and 22 is the vibrating filter assembly comprising a plurality of active discs 32 and a pair of end discs 33 and 34, respectively.

The discs 32, 33, and 34 are connected together by longitudinal coupling wires 36 which are attached to the peripheries of the disc. The discs are spaced equidistant apart.

As pointed out in the Doelz application, Serial Number 248,011, the thickness and diameter of the discs determines the resonant frequency of the filter.

The cylinders 21 and 22 are formed with openings designated as 37 in Figure 2 and the end discs 33 and 34 are received therein. These discs are firmly attached to the cylinder by welding.

Thus, the end discs 33 and 34, respectively, are firmly supported by the cylinders 21 and 22 and do not comprise a part of the vibrating system. The spacing between the disc 33 and the adjacent active disc is substantially the same as that between a pair of active discs. The short lengths of coupling wire joining the active filter structure and the inactive end discs act as springs interspaced between the vibrating filter structure and the fixed mounting structure. An opennig is formed through the lower edge of the discs 33 and 34. A driving rod 39 extends through the opening 38 and is attached to the periphery of the active disc 32. The rod 39 extends through the driving coil 23. g

The active disc 32 at the opposite end of the filter also has a driving wire 41 that extends within the confines of the cylinder 22 and through a second driving coil contained therein. The output leads of the driving coil within the cylinder 22 are designated as 42 and 43, re-

' spectively, and are connected to contacts 44 and 46, re-

Figure 4 is the response curve of the modified filter according to the present invention;

Figure 5 illustrates the normal disc and its nodal lines;

Figure 6 illustrates a conventional disc with its vibration pattern;

Figure 7 illustrates a disc according to the present in vention with its nodal pattern; and

Figure 8 illustrates a secondary vibration pattern of the disc shown in Figure 7.

Figure l is a view of the filter with the top cover removed and illustrates a base plate 10 upon which are mounted a pair of stand-cits 11 and 12, respectively. As best shown in Figure 2, the stand-offs 11 and 12 are each formed in two parts with lower parts 13 and 14, respectively, and top parts 16 and 17 respectively. The lower parts 13 and 14 are attached to the base plate 10 by set screws 18 and the top parts 16 and 17 are attached by set screws 19 to the lower parts.

At the junction between the upper and lower parts of the stand-offs 11 and 12 are formed circular openings lined with a suitable gasketing material through which cylinders 21 and 22 extend. These cylinders are clamped by the stand-offs 11 and 12. The cylinders are hollow as shown in cut away in Figure 2 and contain therein a magnetic driving coil 23 which has output leads 24 and 26 which are attached, respectively, to output contacts 27 and 28. Contacts 27 and 28 extend through the base plate 10.

Polarizing magnets 29 and 31, respectively, are supspectively, which extend through the base plate 10.

A cover member 47 is generally rectangular in shape and has downwardly extending sides 48. It may be placed over the filter assembly and firmly attached to the base plate 10, as for example, by crimping or soldering. The filter unit may be hermetically sealed if desired.

Attached to the base plate are a pair of mounting pins 49 and 51 which may be used to attach the filter assembly to a chassis. Input and output leads may be connected to the terminals 27, 28, 44 and 46.

Thus, the apparatus illustrated in Figures 1 and 2 is an electromechanical filter which forms the subject matter of other patent applications.

Referring particularly, however, to the discs 32, it has been discovered that the normal mode of vibration is as shown in Figure 5, where the disc vibrates in diaphragm fashion with maximum velocities occurring at the center, at the edge of the disc, and intermediate between these points and with a pair of nodal rings 52 and 53 as shown. For example, a particular disc might be resonant at 455 kilocycles in the mode shown in Figure 5. There may be present, however, in the same disc a mode of vibration such as shown in Figure 6, wherein a single nodal circle 54 and a p air of nodal lines 56 and 57 exist. Such a vibration pattern might appear, for example, at 415 kilocycles.

The resulting response from a filter is shown in Figure 3 wherein the desired frequency response of the disc is centered at 455 kilocycles and an undesired response occurs at 415 kilocycles with a level of 15 decibles be low the response at 455 kilocycles. Thus, if all the discs 32 of the filter had the response shown in Figure 3, there would be an undesired response at 415 kilocycles at a level 15 decibles below the 455 kilocycles response.

Figure 7 illustrates a disc 58 which has a hole 59 drilled 3 through its center. It has a normal mode of oscillation at 455 kilocycles, as shown in Figure 7, with a pair of nodal circles 61 and 62.

It is discovered that the disc shown in Figure 7 has a secondary mode of vibration at 425 kilocycles. The" 415 kilocycle spurious response is 40 decible's below the response at 455 kilocycles. This is shown in the curve of Figure 4. i

The end active discs 58 in the assembled filter can be drilled with a center opening 59, such as shown in Figure 7, and all of the discs tuned to a normal mode of oscillation of 455 kilocyles. The end discs 58 will have a spurious response at 425 kilocycles, whereas the central discs which have not been drilled will have a spurious response at 415 kilocycles, and since the center discs cannot couple vibration at 425 kilocycles, this spurious response will be eliminated by them. Alternatively, since the end discs have a spurious response of 425 kilocycles, they will substantially remove all spurious response at 415 kilocycles and therefore only the desired signal or 455 kilocycles will be transmitted.

It is seen that this invention provides means for substantially eliminating spurious response by varying the secondary vibration modes of various discs in an electromechanical filter so that even though all the discs are tuned to a common normal mode, the secondary modes are different and will therefore be eliminated by the filter.

Although this invention has been described with respect to a particular embodiment thereof, it is not to be so limited as changes and modifications may be made therein which are within the full intended scope of the invention, as defined by the appended claims.

I claim:

1. In an electromechanical filter, a resonating assembly comprising a plurality of discs mounted in parallel spaced relationship, a plurality of longitudinal coupling wires attached to the peripheries of said discs, en-d supporting means at either end of said resonating assembly and attached to the ends of said longitudinal coupling wires to support the resonating assembly, each of a portion of said discs formed with an opening therethrough the remaining discs having no openings, all of said discs tuned to the same primary frequency, and the discs formed with an opening therethrough having a diiferent spurious response than the remaining discs.

2. Means for eliminating spurious frequencies in an electromechanical filter comprising, a resonating assembly having a plurality of discs mounted in parallel spaced relationship, a plurality of longitudinal supporting wires attached to the periphery of said discs, end supporting means at either end of said resonating assembly, the ends of said longitudinal supporting wires attached thereto to support said resonating assembly, all of said discs tuned to the same primary frequency, each of a portion of said discs formed with an opening therethrough and the remaining discs with no openings so that the spurious response of the portion of said discs formed with an opening will be different from the spurious response of the remaining discs with no openings.

3. An electromechanical filter, a resonating assembly comprising a plurality of discs mounted in parallel spaced relationship, longitudinal coupling wires attached to the peripheries of said discs, end supporting means attached to said longitudinal coupling wires to support the resonating assembly, all of said discs tuned to the same primary frequency, each of a portion of said discs formed with a central opening therethrough and having spurious responses at certain frequencies other than the primary frequency, and the remaining of said discs having no central opening and having spurious responses which are different from those of the discs formed with a central opening.

References Cited in the file of this patent UNITED STATES PATENTS 2,276,013 Bohannon Mar. 10, 1942 2,615,981 Doelz Oct. 28, 1952 2,652,543 Anthony Sept. 15, 1953 2,656,516 Doelz Oct. 20, 1953 2,694,186 Kinzer et al. Nov. 9, 1954 2,709,243 Babcock May 24, 1955 

